Most cost-effective way of addressing thousands of bright LEDs

[.o:0|O|0:o.]

I am at the beginning of a design, which I hope to have ready for Christmas. In principle it is simple, in practice it is a bit of a headache already.

I need to program a microcontroller, a PIC in this case, to flash >5000 bright LEDs in sequence of predetermined combinations.

The number of predetermined combinations is large (>1000) so unless there is a particularly efficient way of using the memory of a PIC, I will probably need to include some form of external memory so that I can transfer the data from a PC to the device in its entirety.

My first thought is to use a PIC to supply data to some yet unknown arrangement of multiplexers, which will then flash the LEDS, via transistors once in each predetermined manner.

The design should be able to flash any number of LEDs at once, or in tight succession if this helps make more efficent use of the batteries and hardware. Batteries might be any number of AAs.

The LEDs might be pulsed with current above their specification, for extra lumens, briefly and infrequently considering that each combination will be different from the last.

One possible way I was thinking about would be to have multiple levels of multiplexers to account for the many independent outputs, but when you consider the software complexity of controlling LEDs through a number of layered multiplexers it might be more convenient to design the software to replace a layer or two, especially since the LEDs of each combination may be flashed in rapid succession.

Ideally I want each update to be as quick as possible. I was thinking that the problem might be in some ways similar to addressing pixels on a TFT screen. Does anyone know if there is a really good way of accomplishing this sort of task?

Thanks in advance!

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[.o:0|O|0:o.]

By the way I had also thought of having a master-slave setup with multiple MCUs, but for that number I would need many MCUs. The number of LEDs is so large that I would still need some other method realistically....

[sonicj]

maxim has a nice application note on large led displays. (see .pdf attached)

i believe mikeselectricstuff on the forum here is an authority on the matter.

[JohnS_AZ]

Okay, I don't want to be a wet blanket but back up a second ...

">5000" ... "any number of AA batteries"

Say that worst case only 10% of the LEDs can be on at the same time. That's 500 LEDs.
Presuming an average of 20mA per LED, that adds up to 10 AMPS.
You need to be considering a car battery, not AA cells.

Suppose, God forbid, you want all 5,000 on at the same time ... that's 100 amps.
Now we're talking a couple of golf cart batteries to get any run time out of the thing.

Sure you could mux them, but you'll take a big hit in brightness.

What's more, you said bright LEDs which could easily draw much more than 20Ma depending on the device.

I think you need to do a little power budgeting  before diving into what are going to be some serious computer horsepower issues.

[.o:0|O|0:o.]

maxim has a nice application note on large led displays. (see .pdf attached)

i believe mikeselectricstuff on the forum here is an authority on the matter.

Thanks for that. Any help is great. Going through it now!

[.o:0|O|0:o.]

Okay, I don't want to be a wet blanket but back up a second ...

">5000" ... "any number of AA batteries"

The batteries are not a strict requirement. Power adapters could be used in the limit if I couldn't devise a suitable parallel/ serial configuration. I should have specified this, but I didn't want to get side tracked......

Since the device will only update (probably 33% to 50% of the LEDs) once every few minutes, I was also considering charging and discharging caps....

The forward current of the LEDs would be about 50mA each. Essentially, they should be ON and OFF as quickly as visibly possible.

Your point is taken, but I am hoping to find a creative way around these issues.

Note that activating all LEDs simultaneously is not a requirement, smaller clusters could be updated in succession as long as the overall update didn't take too long. You can find AAs apparently rated at 2500mAh Nickel Metal Hydride. Even if these ratings are overstated, 15 AA's is not an inconsiderable amount to play about with.

If I can't find a way of delivering the current I need in the short time required, I will be forced to reduce the number of LEDs, but consider this, if I flashed clusters of 50 x 50mA at a time, on paper that is 2.5A

Thanks for the link...

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[.o:0|O|0:o.]

I just had a look at that Maxim datasheet and I think there is a good chance I will be using their devices.

I may be able to drastically reduce the number of LEDs to 8x8 x 8 = 512. However, I would still be seeking to flash, though very briefly, as many LEDs at once as I could (or in clusters or sequentially, in as brief a time as possible).

Thanks again.

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[Simon]

1) your rambling and probably trying to do something beyond your abilities
2) your still not understanding just the amount of power you are asking for, you have a big power management project before you even start on the control system.
3) in view of 2&3 why not give us full details of the desired result as you seem to have a goal in mind but not quite know how to deal with the scale on which you are trying to do it.

[qno]

search for charlieplexing

[allanw]

I just had a look at that Maxim datasheet and I think there is a good chance I will be using their devices.

I may be able to drastically reduce the number of LEDs to 8x8 x 8 = 512. However, I would still be seeking to flash, though very briefly, as many LEDs at once as I could (or in clusters or sequentially, in as brief a time as possible).

Thanks again.

.o:0|O|0:o.

Be careful, Maxim IC's are insanely expensive.

Look into the XMOS microcontrollers. Their parallel execution is perfect for something like LED matrix driving. They even have a bunch of calculations done already for how many XMOS's are needed for a given amount of LED's.

Maybe look into the TLC5947 (if you want fading), or the TLC5951? I did a project with them. It should be pretty straightforward to expand it by using a lot more drivers and multiple microcontrollers. https://www.eevblog.com/forum/index.php?topic=1487.0

It's going to get pretty expensive though.

Another idea is to use an FPGA, which is perfect for this sort of task... massively parallel I/O.

[Simon]

maxim also have a tendency to not have parts available, I once bought out farnell of chips with just 12 parts although suspect that they were going to ditch the line in favor of a newer product of which they still only carried 50 odd when I last ordered

[.o:0|O|0:o.]

Thanks for all the advice. I wanted to avoid FPGAs, but I will now have reconsider all my options. It is interesting how the simple task of flashing a LED becomes many times more complicated with increased numbers...

Nice circuit by the way!

Fortunately I don't have the added requirement of variable brightness and the LEDs themselves are unlikely to warm up significantly because of their brief ON times. I will probably need to cool my chips if I find a particularly efficient method. The TLC5951 appears to provide 40 mA per channel, I might look into it. Farnell don't have it at this time though.

Interesting reading about the charlieplexing... I think I will continue to browse the IC section for ideas.

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[Jon Chandler]

Your point is taken, but I am hoping to find a creative way around these issues.


.o:0|O|0:o.

Finding a creative way around the laws of physics is difficult at best! 

[Simon]

Your point is taken, but I am hoping to find a creative way around these issues.


.o:0|O|0:o.

Finding a creative way around the laws of physics is difficult at best! 

sure is, physics is the bottom line in any science and technology. there are many things about electronics, many quirks that are simply down to the way materials we make parts with work on a physical level.

[allanw]

This is more of a systems engineering problem than overcoming device physics though...

[Simon]

yes it is. really it's down to maths and the amount of combination you can implement with as little address lines as possible

[Mechatrommer]

It is interesting how the simple task of flashing a LED becomes many times more complicated with increased numbers...

its not that complicated, as long as you have enough fund for it.
i agree with allanw, this is engineering, not science. science is to find the truth, and engineer is to get around it or in more precise word... manipulating it

[.o:0|O|0:o.]

Finding a creative way around the laws of physics is difficult at best! 

...So you believe that flashing a large number of LEDs within a finite period is impossible ~against the laws of physics~, even if modest numbers are flashed at a time to circumvent the lack of an ideal power supply (calculated based on the charge available at any given time - which might be stored and discharged by an array or capacitors)?

Above I suggested a number of clusters of fifty 50 mA LEDs flashing briefly one cluster after the other (2.5 A per cluster). You reckon that this is ~against the laws of physics~?

I don't believe it is impossible. I believe that a level of complexity exists in addressing such a large number of LEDs (which I have brought down to a more manageable 512), that there are trade-offs to be made in several areas (including power), that the cost could be high or even prohibitive ....hence the "Most cost-effective" in the title.

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[.o:0|O|0:o.]

yes it is. really it's down to maths and the amount of combination you can implement with as little address lines as possible

........Well, its a start! (rolls eyes)

[Jon Chandler]

The laws of physics have to do with the amount of energy in a few AA batteries.  There's only so much energy there no matter how you slice it.  A short burst of high current, a longer period of moderate current - the amount of energy available doesn't change (much).

The mA-hr rating of a battery doesn't say how much current a battery can supply.  It's a measure of the energy in the battery.  At high current draws, the rating goes down.  

As was stated earlier, you should start with some calculations on current draw.

[allanw]

Putting a capacitor across the battery terminals is enough to supply those short spikes of current. It will be too hard to notice anyway.

[.o:0|O|0:o.]

The laws of physics have to do with the amount of energy in a few AA batteries.  There's only so much energy there no matter how you slice it.  A short burst of high current, a longer period of moderate current - the amount of energy available doesn't change (much).

The mA-hr rating of a battery doesn't say how much current a battery can supply.  It's a measure of the energy in the battery.  At high current draws, the rating goes down.  

As was stated earlier, you should start with some calculations on current draw.

You have transformed my hypothetical 15 AA batteries into "a few AA batteries". 2400 mAh per AA is an indication of the capacity and longevity of a battery. Batteries have an output resistance which means they cannot safely supply more than a certain amount of current at a time, each. However, arranged in series and parallel, the drain on each battery is divided.

e.g.

2.5 A / 7 = 334 mA per battery pair (2 x 1.5V) per cluster flash.

Apparently, AAs are more than capable of sustaining this sort of drain: http://www.candlepowerforums.com/vb/showthread.php?t=79302

And I haven't even touched on the ancillary use of capacitors yet.

Peace!  

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[.o:0|O|0:o.]

Well, according to the graphs, not quite 1.5V at those currents, but that is why I doubled up...

[Feanor]

Might also want to check out 3D LED cubes. Another application involving thousands of LED's. Some are three color tripling the number to be driven.

[.o:0|O|0:o.]

Might also want to check out 3D LED cubes. Another application involving thousands of LED's. Some are three color tripling the number to be driven.

Thanks for that! I came across LED cubes a while back and had totally forgotten about them. Like this:

http://technabob.com/blog/2008/04/06/led-3d-cube-color-display/  

In all probability they used the Maxim chips suggested above, but maybe not.

(Phew, at least some people have a clue!)


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PS.
This one exceeds anything I want to achieve. And yet................
http://www.seekway.com.cn/e/3d/a08/detail.htm